In an RF communications system, an RF power amplifier feeds an RF output signal to an antenna for transmission. The delivered power to the antenna may be affected by an impedance match between the RF power amplifier and the antenna. Impedance mismatches may cause reflections of the RF output signal, thereby reducing the delivered power to the antenna. There are several possible causes of impedance mismatch. In a multi-mode or multi-band RF system, the antenna may need to be coupled to multiple RF power amplifiers, to multiple RF receivers, or the like. As a result, providing a matched impedance to the antenna under all operating conditions may be problematic.
Notably, the input impedance to the antenna may change due to localized antenna conditions. For example, in a portable RF terminal, such as a cell phone, anything in proximity to the antenna may change the antenna's input impedance. Holding a cell phone next to a user's body or laying the cell phone on a table may change the input impedance, thereby causing impedance mismatches. Experimental data has shown that normal body movements can cause significant antenna input impedance changes in time frames on the order of one millisecond. One technique for correcting impedance mismatches is to use a tunable antenna matching circuit between the RF power amplifier and the antenna to provide an impedance match. Power detection circuitry may be coupled in the signal path between the RF power amplifier and the antenna to determine forward power from the RF power amplifier and reflected power back to the RF power amplifier to assess the impedance mismatch. Then, the tunable antenna matching circuit may be adjusted to improve the impedance match, thereby decreasing the reflected power and increasing the delivered power to the antenna.
During RF communications, a first RF terminal may be communicating with a second RF terminal. The second RF terminal may send ongoing commands to the first RF terminal instructing the first RF terminal to adjust its output power based on signal strengths of the signals received by the second RF terminal from the first RF terminal. Such ongoing commands may be part of an output power control loop between the first RF terminal and the second RF terminal. By continually adjusting the output power from the first RF terminal, the signal strengths are large enough to provide reliable communications, while reducing interference caused by the signals transmitted from the first RF terminal that reach other RF communications devices.
The signal strengths of the signals transmitted from the first RF terminal are directly related to the delivered power from an RF power amplifier to an antenna in the first RF terminal. As such, adjustments made to the tunable antenna matching circuit may influence or cause errors to the output power adjustments of the first RF terminal in response to commands from the second RF terminal. Thus, there is a need to reduce or compensate the influence that tunable antenna matching circuit adjustments have on the output power adjustments of the first RF terminal in response to commands from the second RF terminal.